Slice 4: super-draft body editing per §9.5 + §9.6

The §17 routing-collapse rule lands in api_branches.py and
api_prs.py — every branches/<branch>/... and prs/<n>/... route
dispatches on the entry's state to pick the right Gitea repo, and
the body extracted from the entry's frontmatter envelope is what
the editor and the diff see. The bot grows open_metadata_pr;
cache grows refresh_meta_branches. Two §17 routes added:
start-edit-branch and metadata. The §9.4 super-draft view replaces
RFCView.jsx's Slice 2 placeholder; a metadata pane modal opens
from the breadcrumb. Branch naming uses edit-<slug>-<6hex> to
dodge the §19.2 path-routing candidate while preserving §9.5's
structural shape.

Covered by tests/test_super_draft_vertical.py (10 tests). The
full Slices 1-4 suite is 35/35 green.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/bot.py

@@ -221,6 +221,65 @@ class Bot:
             pr_number=pr_number,
         )
 
+    # ----- Meta repo: metadata-pane PRs (§9.5) -----
+
+    async def open_metadata_pr(
+        self,
+        actor: Actor,
+        *,
+        org: str,
+        meta_repo: str,
+        slug: str,
+        new_file_contents: str,
+        prior_sha: str,
+        pr_title: str,
+        pr_description: str,
+    ) -> dict:
+        """Per §9.5: a metadata-pane edit (title or tags) on a super-draft
+        opens a tiny meta-repo PR that touches only the frontmatter of
+        `rfcs/<slug>.md`. One commit, one PR, easy to triage. The branch
+        name uses the dash-separated `metadata-<slug>-<6hex>` shape — same
+        routing-friendly form Slice 4 picked for edit branches per the
+        §19.2 path-routing candidate.
+        """
+        import secrets
+
+        branch = f"metadata-{slug}-{secrets.token_hex(3)}"
+        await self._gitea.create_branch(org, meta_repo, branch, from_branch="main")
+        commit_subject = pr_title
+        commit_message = _stamp_single(commit_subject, actor)
+        result = await self._gitea.update_file(
+            org,
+            meta_repo,
+            f"rfcs/{slug}.md",
+            content=new_file_contents,
+            sha=prior_sha,
+            message=commit_message,
+            branch=branch,
+            author_name=actor.display_name,
+            author_email=actor.email or f"{actor.gitea_login}@users.noreply",
+        )
+        commit_sha = result.get("commit", {}).get("sha") or result.get("content", {}).get("sha") or ""
+        _subject, pr_body = _stamp("", pr_description, actor)
+        pr = await self._gitea.create_pull(
+            org,
+            meta_repo,
+            title=pr_title,
+            body=pr_body,
+            head=branch,
+            base="main",
+        )
+        _log(
+            actor,
+            "open_metadata_pr",
+            rfc_slug=slug,
+            branch_name=branch,
+            pr_number=pr["number"],
+            bot_commit_sha=commit_sha,
+            details={"pr_title": pr_title},
+        )
+        return pr
+
     # ----- Per-RFC repo: branches (§8.3, §8.14) -----
 
     async def cut_branch_from_main(